The present invention relates to a cladding member and, more particularly, a cladding member for double-clad fiber lasers and amplifiers. In addition, the present invention relates to an optical fiber formed with such a cladding member.
Fiber lasers and amplifiers can be made from optical fibers whose cores are doped with rare-earth materials. Optical fibers are pumped with light of a suitable wavelength to achieve laser action or signal amplification. The laser transition wavelength depends on a selection of rare-earth materials and host composition. Various rare-earth ions including Nd, Er, Yb, Ho, Tm, Pr, Eu, Ce, Dy, Sm, Tb, Gd and Pm have been used, which can provide laser action.
Based on the pumping technique, fiber lasers are classified into core-pumped fiber lasers and clad-pumped fiber lasers. In a core-pumped fiber laser, light from a pump source, such as a diode laser, is coupled directly into the fiber core which is doped with rare-earth materials. In a clad-pumped fiber laser, the single mode core doped with rare-earth materials is surrounded by an undoped multimode cladding. The pump light is launched into the cladding and guided in the cladding by a second cladding (glass or a polymer) with a lower refractive index. Cladding-pumped fiber lasers are therefore also referred to as double-clad fiber lasers. The main advantage of clad pumping over core pumping is that clad-pumped fibers are readily scalable to high output powers.
FIG. 1 shows a conventional double-clad fiber in which the core 2 and the first cladding 4 are both circular and are concentric with each other. Such a circular cladding 4 can support many modes which spiral around the core 2 without asserting any intensity on the core 2. Moreover, light launched into a circular cladding 4 will not be absorbed fully by the core 2. Thus, the circular-type double cladding 4 precludes efficient coupling of the pump light from the cladding to the core 2.
Other cladding members 4 are used to facilitate pump light rays to cross to the core 2 as they travel along the length of the fiber. FIG. 2(a) shows a double-clad fiber that has an off-centered core 2 as disclosed in U.S. Pat. No. 4,815,079. By off centering the core 2, skew mode light, which does not intersect the core 2 in a concentric geometry, can be made to intersect the core 2. However, an off-center-type cladding member 4 is still inefficient and impractical because a fraction of the skew mode light fails to intersect the core 2. Moreover, an off center type cladding member 4 is not amenable for use with standard fibers because it is difficult to align and line the off centered core 2 with the standard core. Any misalignment of the cores can lead to significant loss of power.
FIG. 2(b) shows another conventional double clad fiber in which a core 2 is centered in an elliptical cladding 4. This elliptical-type cladding 4 would be most efficient if the core 2 is positioned at one of the ellipse foci. However, such a cladding 4 is difficult to make, especially when the core 2 is to be positioned at an ellipse focus. Moreover, this elliptical type cladding 4 is not compatible with standard circular fibers, to which it has to be spliced.
FIG. 2(c) shows a polygon-type cladding 4 as disclosed in U.S. Pat. No. 5,533,163 wherein the polygons are categorized as xe2x80x9cconvex polygonsxe2x80x9d having the property that, if a plurality of such polygons are used to tile a plane surface, all of them will fit into the tiling with no spacing left between adjacent polygons. Further, all the polygons will be mirror images of one another about any common side. Accordingly, the above property limits the polygons to three (3), four (4) and six (6) sided polygons. These polygonal shapes significantly differ, in cross-section, from those of circular type fibers, which are commonly used to deliver pump power to double clad fibers. Therefore, polygon type cladding 4 results in a large mismatch area with a circular fiber causing inefficient coupling of pump light into double clad fibers.
FIG. 2(d) shows a double clad fiber with a D-shaped cladding 4 as disclosed in U.S. Pat. No. 5,864,645. Comparing to that of the polygon type cladding 4, a D-shaped cladding 4 has a smaller portion of cladding 4 that is removed. Therefore, a D-shape cladding 4 has a substantially circular shape that can effectively facilitate the double clad fiber to splice with a circular pump delivery fiber. However, as a general principle, a larger amount of cladding 4 is to be removed from the cladding 4 to improve the coupling efficiency. To solve the above dilemma, a much longer fiber is needed to couple the required pump light from the cladding 4 to the core 2 without losing the advantage of a substantially circular cladding 4. Such an extra coupling length is inefficient.
Therefore, it is desired to obtain a novel cladding member that facilitates both efficient coupling and effective splicing. Further, it is desired that such a novel cladding member as well as the optical fiber formed with the novel cladding member are easy to manufacture. The present invention provides a cladding member and an optical fiber that meet all the above requirements.
The present invention provides an optical fiber for use in fiber lasers and amplifiers wherein the optical fiber has a core member surrounded by a cladding member for receiving pump energy and transferring the pump energy to the core member. The optical fiber also has an outer layer surrounding the cladding member. The cladding member has a circular exterior periphery and a predetermined refractive index (nc). The cladding member has an index modified region that directs light to the core member. The index modified region has a stress field portion with a predetermined refractive index (ns). The difference between the refractive index of the cladding member and that of the stress field portion (ncxe2x88x92ns) is within such a range that the stress field portion does not affect the polarization properties of the light traveling in the core member. Preferably, the difference between the refractive index of the cladding member and that of the stress field portion (ncxe2x88x92ns) is less than 10xe2x88x924 , and more preferably 10xe2x88x925.
In a preferred embodiment, the modified index region contains one or more dopants of any combination of elements Ge, Al, P, B and F. More preferably, the modified index region contains one of the following: (1) Ge and Al; (2) Ge and P; (3) Ge and B; (4) Ge and F; (5) P and Al; (6) Ge, P and Al; (7) Ge, P and B; (8) Ge, P and F; and (9) free air space.
The present invention also relates to a clad article used in optical fibers for receiving pump energy and transferring the pump energy to a core member. The clad article has a core member. A cladding member surrounds the core member and has a polygon shape of at least eight sides. Preferably, the clad article has a polygon shape of eleven sides or more. It is preferred that the clad article has a polygon shape of twelve sides or less.
The present invention further relates to a clad article used in optical fibers for receiving pump energy and transferring the pump energy to a core member. The clad article has a core member. A first cladding member surrounds the core member and has a substantially circular exterior periphery. The first cladding member has a cut-out portion extending from the exterior periphery and terminating with a curved boundary in the first cladding member. The clad article also has a second cladding member surrounding the first cladding member. The second cladding member has an interior periphery complementary to the exterior periphery of the first cladding member. In a preferred embodiment, the curved boundary of the cut-out portion is concave in relation to the circular exterior periphery. The first cladding member has three cut-out portions evenly distributed along its circular exterior periphery.